Add an append_file_content function

Add content to a file resource.

This allows to generate concatenated files on the master side, without managing multiple file resources.

The first argument is the file path. You are responsible for creating the file resource yourself prior to calling the function. This way, you can finely control the file's parameters.

The second argument is the content to be added to the file.

The third (optional) argument is the order, specifying the position in which the content should be added. The default order is '10'. If you specified a content in the original file resource, this content is kept with order '00'

Examples:


    file { '/tmp/target':
      content => "Orig content\n",
    }

    append_file_content('/tmp/target', "hello, ")
    append_file_content('/tmp/target', "world\n")
    append_file_content('/tmp/target', "This is a multi
    line statement
    ", '01')
    append_file_content('/tmp/target', template('foobar/example.erb'), '05')

- *Type* : statement

Yes, the way the 3x functions work is that they are loaded into anonymous modules (managed by `Puppet::Parser::Functions`), then they are placed on the scope instances by https://github.com/puppetlabs/puppet/blob/master/lib/puppet/parser/scope.rb#L897-L901.